This invention relates to shadow mask type cathode ray tubes and, particularly to means for supporting a shadow mask within such tubes.
In a shadow mask tube, a plurality of convergent electron beams are projected through a multiapertured color selection shadow mask to a mosaic screen. The beam paths are such that each beam impinges upon and excites only one kind of color-emitting phosphor on the screen. Generally, the shadow mask is attached to a rigid frame, which in turn, is suspended within the picture tube envelope.
Presently, all commercial color picture tubes have a front or viewing faceplate portion that is either spherical or cylindrical. However, it is desirable to develop a tube having a generally flat faceplate. According to prior art tube design concepts, in tubes having curved faceplates, the shadow mask is similarly curved so that it somewhat parallels the faceplate contour. Thus, in keeping with these prior art concepts, in a tube with a flat faceplate, the corresponding shadow mask should also have an almost flat contour. However, such a mask has insufficient self-supporting strength or rigidity. One way to provide this strength or rigidity would be to put the mask under tension as is done in some commercially available tubes having cylindrical faceplates. However, tension methods require undersirable heavy and expensive frame structures. Another recently suggested way of providing strength to the mask is to give it some degree of contour. But in addition to deviating from prior design concepts, this method raises an additional doming problem. The problem of doming occurs during an initial period of tube operation. It is caused by shadow mask heating and expansion when the mask is bombarded by the electron beams.